The present invention relates to a hydraulic system comprising a first pump that delivers hydraulic fluid to a first load outlet point and to a second load outlet point, a second pump that delivers hydraulic fluid to the second load outlet point, but not to the first load outlet point, means for detecting the load pressure at the first load outlet point and means for detecting the load pressure at the second load outlet point.
The invention also relates to a work machine provided with a hydraulic system according to the invention.
The invention is generally applicable for all machines that comprise at least two pumps, where each of the pumps is arranged to deliver hydraulic fluid to a common load outlet point, but where only one of the pumps delivers hydraulic fluid to an additional load outlet point.
It should be noted that a load outlet point can, in turn, comprise a plurality of load outlet points.
In work machines of the loader type designed according to previously-known technology, there are hydraulic systems with a pump that delivers hydraulic fluid for both the steering of the work machine and for one or more work functions, such as raising and lowering of a lifting unit, for example a bucket, and also a pump that only delivers hydraulic fluid for the said work functions. The output of the first pump, or rather the pressure that it is to provide, is controlled on the basis of signals relating to the detected load for the steering function and on the basis of signals relating to the detected work load, that is the load associated with the said work function or functions. In addition, the system comprises a priority valve that ensures that the flow from the first pump to the steering function is given priority over the flow to the work function or functions.
The output of the second pump, or more specifically the pressure that it is to provide, is also controlled on the basis of signals relating to the detected load for the steering function and the detected work load.
It is desirable to provide a hydraulic system that makes it possible to minimize the power that the hydraulic system requires, at least in certain operating situations, in a work machine or the like that is provided with such a hydraulic system, in order in this way to make as large a part as possible of the output that is taken momentarily from the motor of the work machine available for other functions than those that the hydraulic system requires.
An example of such another function is the propulsion of a work machine. The smaller the power that is required for driving the hydraulic system's pumps at a given moment, the greater the power that can be utilized, for example, for increasing the speed of the motor of the work machine, which can be required, for example, for rapid acceleration of the work machine.
According to an aspect of the present invention, a hydraulic system comprises means for controlling the first pump on the basis of the detected load pressure at the first load outlet point and the detected load pressure at the second load outlet point, and means for controlling the second pump on the basis of the detected load pressure at the second load outlet point, but not on the basis of the detected load pressure at the first load outlet point.
According to another aspect of the present invention, a work machine is provided with a hydraulic system according to the invention.
The first load outlet point is typically associated with the steering function of the work machine, while the second load outlet point is associated with one of the work machine's work functions.
The work machine has preferably a lifting unit, and accordingly the work function is a raising, lowering or tilting function of the lifting unit.
Further advantages and characteristics of the invention will be apparent from the following detailed description.